Method and apparatus for producing low sulfur metallized pellets

ABSTRACT

An improved method and apparatus for forming metallized iron by direct reduction of particulate iron oxide is disclosed. Spent reducing gas is recycled from the reduction furnace through a cooler-scrubber and a catalyst-containing stoichiometric gas reformer. Upon removing the process gas from the cooler-scrubber, it is contacted with a chlorine dioxide spray, then compressed and cooled, the sulfur compound removed, and the process gas recycled either into the furnace cooling zone, or directly into the reformer, or divided and directed into both uses. Thus, most of the sulfur containing components of the spent reducing gas are removed, thereby reducing the sulfur contamination of the gas reformer catalyst. Reducing sulfur contamination of the gas reformer catalyst improves the overall efficiency of the direct reduction process.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for the removalof hydrogen sulfide from an H₂ S-laden process gas stream in the directreduction of metallic oxides, and producing a metallized product havinga very low sulfur content. The invention is particularly useful in thedirect reduction of iron.

BACKGROUND OF THE INVENTION

Direct reduction processes for producing metallized iron from iron oxidepellets, lump ores, or similar materials which include iron oxide, arewidely known and used in the steel industry. Suitable direct reductionprocesses for forming metallized iron are disclosed in U.S. Pat. Nos.3,128,174 and 3,881,916. In a direct reduction process such as theMidrex direct reduction process, pellets of metal oxides and/or lumpores are charged into the top of a shaft furnace to establishgravitational flow therethrough. Reducing gas consisting essentially ofcarbon monoxide and hydrogen is heated to sufficient temperature toeffect direct reduction, is introduced to the metal oxide burden in thefurnace, ascends through the burden in counter-flow relation to thegravitationally descending burden, and reacts with the burden, formingmetallized pellets and a reacted top gas consisting principally ofcarbon dioxide and water along with some unreacted reductants, which aredrawn off as sulfur-containing spent top gas. The resulting hotmetallized product descends into the cooling zone of the furnace whereinit is cooled by contact with cool gas injected into the cooling zone.Cold direct reduced product is discharged from the bottom of thefurnace.

Additionally, U.S. Pat. No. 3,748,120 discloses a shaft furnace directreduction process in which spent reduction gas is catalytically reformedfrom a mixture of gaseous hydrocarbon and spent reducing gas from thereduction process. In this process the spent reducing gas is cleaned andcooled upon exiting the reduction furnace and prior to being introducedinto a catalyst containing reformer. In the direct reduction process thereducing gas passes through and reacts with the burden to produce sulfurcontaining gases which contaminate the catalyst. Sulfur contamination ofthe catalyst lowers the overall efficiency of the process. Processes forreducing the sulfur level in the recycled spent reducing gas byincreasing the metallic ion content of the scrub water utilized by thecooler-scrubber are also known.

The present invention provides an improved method and apparatus forreducing the sulfur contamination of the gas reformer catalyst byremoving sulfur compounds from the spent process gas stream by reactionwith chlorine dioxide prior to reintroduction of the process gas intothe hot gas reformer.

DESCRIPTION OF THE PRIOR ART

Applicant is aware of the following U.S. Patents concerning the use ofchlorine dioxide.

    __________________________________________________________________________    U.S. Pat. No.                                                                        Issue Date                                                                          Inventor                                                                           Title                                                       __________________________________________________________________________    4,430,228                                                                            02-07-1984                                                                          Paterson                                                                           PROCESS FOR REMOVING                                                          DISSOLVED IRON FROM WATER                                   4,451,361                                                                            05-29-1984                                                                          Paterson                                                                           APPARATUS FOR REMOVING                                                        DISSOLVED IRON FROM WATER                                   4,629,502                                                                            12-16-1986                                                                          Sherman                                                                            PRESSURIZED REACTION OF                                                       REFRACTORY ORES WITH HEAT                                                     RECOVERY                                                    4,902,408                                                                            02-20-1990                                                                          Reichert                                                                           PROCESS FOR REMOVING                                                          HYDROGEN SULPHIDE USING                                                       METAL COMPOUNDS                                             4,941,917                                                                            07-17-1990                                                                          Cenegy                                                                             PROCESS FOR RECOVERY OF                                                       PRECIOUS METALS FROM                                                          CARBONACEOUS ORES USING                                                       CHLORINE DIOXIDE                                            __________________________________________________________________________

Reichert teaches that chlorine dioxide is known for the purpose ofremoving hydrogen sulfide in the processing of oil and oil products inrefineries or petrochemical plants. Reichert states that chloridedioxide is a toxic substance requiring special handling, as well asbeing corrosive and of low efficiency where low hydrogen sulfidecontents are to be treated.

Sherman teaches a pressure hydrometallurgy process in which an orecontaining slurry is oxidized to break the metal-sulfur bonds byintroducing a heated liquid containing oxidizing elements such as oxygenor chlorine dioxide to flow through the slurry. This process producessulfuric acid as a byproduct.

Both of the Patterson Patents utilize chlorine dioxide to oxidizesoluble ferrous iron which has been dissolved in water, such as wellwater.

Cenegy teaches a method for destroying or reacting organic carbon in acarbonaceous ore containing precious metals by forming slurry of the oreand contacting it with an aqueous solution of chlorine dioxide.

In each of the references above, other than Reichert, chlorine dioxideis reacted with an ore of a metallic substance. In the Reichertreference, hydrogen sulfide is removed from oil and oil products.

SUMMARY OF THE INVENTION

The invention provides both a method and apparatus for removing hydrogensulfide from a process gas stream in the direct reduction of metals,particularly iron. In the present invention, hydrogen sulfide containingspent reducing gas from an ore reduction furnace is treated withchlorine dioxide before it comes into contact with metallized product,such as metallized iron material. Chlorine dioxide in aqueous solutionis sprayed into a stream of removed top gas after which the streampasses through a compressor, then the fluids are removed and thesulfur-free stream passes into the cooling zone of the shaft furnace, ordirectly into a process gas reformer.

Upon removing the process gas from the scrubber-cooler, it is contactedwith a chlorine dioxide spray, then compressed and cooled, the sulfurcompound removed, and the process gas recycled either into the furnacecooling zone, or directly into the reformer, or divided and directedinto both uses. Thus, the sulfur containing components of the spentreducing gas are lowered, thereby reducing the sulfur contamination ofthe gas reformer catalyst. Reducing sulfur contamination of the gasreformer catalyst improves the overall efficiency of the directreduction process.

The desulfurizing apparatus consists of a source of sodium hypochlorite(NaOCl), a source of sodium chlorite (NaClO₂), a source of hydrochloricacid (HCl),a source of water, a pump, chlorine dioxide (ClO₂) generator,and injection means such as a nozzle. Control and monitoring devices arealso utilized, as required.

The invented process is applied to either of two direct reduction gasflow arrangements or flow sheets. These are designated (1) the StandardFlow Sheet (SFS), as depicted in U.S. Pat. No. 3,748,120, and (2) theAlternate Flow Sheet (AFS), as depicted in U.S. Pat. No. 3,749,386. Inthe former, cleaned, cooled spent reducing gas is recycled to acatalytic reformer for reforming to reducing gas. In the latter, cooled,cleaned spent reducing gas is introduced to the cooling zone of a shaftfurnace as cooling gas, which also acts to desulfurize the gas, beforebeing introduced to the reformer for catalytic reforming to reducinggas.

Iron ores always contain a small percentage of sulfur in the form ofiron sulfide. During reduction, a portion of this sulfur is liberatedinto the reducing gas stream. In either gas flow sheet, spent reducinggas exiting the top of the furnace contains some H₂ S. In the SFS, thisgas becomes the process gas which is introduced to the reformer whereinit is reformed, generally along with methane, to produce H₂ and COreductants. Reformer catalyst is very sensitive to H₂ S and the SFS useis generally limited to ores containing no more than a maximum of 0.004%sulfur.

In the AFS, spent gas from the reducing furnace containing H₂ S iscooled and passed through the cooling zone of the furnace. Because ofthe affinity of hot iron for sulfur, the cooling direct reduced productstrips H₂ S from the gas, which becomes sulfur-free process gas and isdelivered to the reformer. The removed sulfur is discharged along withthe DRI product. The AFS generally can handle ores containing up to0.007% sulfur without raising sulfur in the direct reduced product to alevel undesirable to steelmakers.

The AFS has greater tolerance for higher sulfur contents and allows theoperator to achieve maximum DRI tonnes/hour production with its gascompressors.

In general, the higher the sulfur content of iron ore, the lower itscost. It is therefore desirable to be able use ores containing moresulfur than the 0.007% that steelmakers will tolerate. Several methodshave been suggested (and some actually tried) to strip H₂ S from processgas without discharging the sulfur with the DRI product, as happens withthe AFS. None of these processes have been cost effective. For example,one such process passes the process gas through a vessel containing ZnO,wherein H₂ S reacts to form ZnS. The stripped process gas then flows tothe reformer. While this process works, its capital cost is high,pressure drop is appreciable, and the ZnS must be regenerated ordisposed of in an environmentally acceptable manner.

The present invention strips H₂ S from the process gas in a processhaving low capital cost, low operating cost, no increase in pressuredrop, and with environmentally acceptable waste products.

OBJECTS OF THE INVENTION

The principal object of the invention is to provide an improved methodof removing sulfur compounds from spent process gas in the directreduction of metal ores.

A further object of this invention is to provide a method for directreduction of sulfur-containing metal ores which results in a low-sulfurmetallized product.

Another object of the invention is to provide apparatus for directreduction of sulfur-containing metal ores which reduces sulfurcontamination of the metallized product.

Another object of the invention is to provide a method and apparatus fordirect reduction of sulfur-containing metal ores which reduces sulfurcontamination of catalyst in an associated stoichiometric reformer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects will become more readily apparent byreferring to the following detailed description and the appendeddrawings in which:

FIG. 1 is a schematic diagram of a vertical shaft furnace and itsassociated equipment for producing metallized iron in accordance withthe preferred embodiment of the invention.

FIG. 2 is another schematic diagram similar to FIG. 1, but illustratingan alternative embodiment of the invention.

FIG. 3 is a schematic diagram of the sulfur removal equipment foroperating and testing the invented process.

DETAILED DESCRIPTION

Although the invention is suitable for use in reduction of other metaloxides, it will be described with regard to iron. Chlorine dioxide gasis a powerful oxidizer. In the present invention, chlorine dioxide isinjected into H₂ S-laden process gas streams of a direct reduction plantto oxidize H₂ S to a water soluble sulfate. The soluble sulfate is thenflushed from the gas stream with water, leaving H₂ S-depleted gas to bereformed, as in the SFS, or to pass through the cooling zone withoutdepositing sulfur on the DRI product prior to reforming as in the AFS.

The sulfate-containing flush water flows into the plant process waterclarifier. Waste products resulting from the gas desulfurizing reactionare discharged with clarifier underflow or with the cooling towercooling water as sulfate (calcium) and NaCl (salt), both of which areenvironmentally benign.

Injecting chlorine dioxide gas into process compressor flush-water isthe preferred way to react ClO₂ gas and H₂ S. Violent agitation withinthe compressor provides intimate contact of the gases, and providessufficient reaction time to quantitatively oxidize all the H₂ S orreduce all the ClO₂, depending on whether H₂ S or ClO₂ is present inexcess. It is desirable to maintain 2 to 5 ppmv H₂ S in the process gas;therefore, less ClO₂ is injected than is stoichiometrically necessary.

FIG. 3 shows the equipment arrangement for injecting ClO₂ in accordancewith the invented process. Tests have shown that ClO₂ injected as aboveremoves H₂ S from the gas stream with no residual ClO₂ remaining in thestream. Neither H₂ S nor chlorine compound gases were detected in thegas beyond the gas cooler, nor downstream of the compressor. This provedthat the process is economic from a chemical cost standpoint. Capitalcost is negligible, only a simple ClO₂ generator is required and nodirect reduction plant equipment changes are needed.

The capability is provided for measuring the H₂ S content of the gasafter exiting the compressor (or the humidifier/cooler 62) andcontroling the rate of ClO₂ injection to maintain a concentration of 2to 5 ppmv H₂ S in the treated gas stream to (1) insure that no residualClO₂ or chlorine compound gases reach the reformer or process gas heatexchangers, and (2) insure that some H₂ S is present in the process gasentering the reformer to protect the reformer catalyst from potentialcarbon degradation. At the compressor water discharge point 60 from thesilencer 58, the discharge water is sampled for chlorine content and forpH as well as for H₂ S. Just beyond the aftercooler 62 is a monitoringpoint D2 for detecting chlorine and H₂ S in the gas stream.

Chlorine dioxide gas is produced as shown in FIG. 3 by reacting sodiumchlorite (bleach), sodium hypochlorite and hydrochloric acid.International Dioxcide, Inc. of Clark, N.J., supplies generators andchemicals to paper mills, dye and other chemical plants to deodorize gasstreams. All these applications involve flue gas, which is highlyoxidizing. Using ClO₂ in the reducing and iron dust-laden gas streampresent in a process for direct reduction of metal oxides, injecting itin the manner set forth above, and causing the reaction to occur withina compressor were all heretofore unknown.

In addition to improving the performances of Midrex direct reductionplants producing cold DRI via either the SFS or AFS methods, theinvention is especially useful for plants producing hot briquetted iron,know as HBI. In the HBI operation, ore is reduced to DRI in thereduction zone of the furnace, as above, but is not cooled. DRI isdischarged hot into briquetting presses to form pillow shaped briquets.In such plants, there is no cooling zone wherein H₂ S could be removedfrom the process gas stream using AFS concept. Therefore, HBI Plants arerestricted to using the more expensive low sulfur ores. The use of thesubject invention to strip H₂ S from process gas to the reformer givesHBI plants an economical way to use cheaper ores.

Referring now to FIG. 1, a vertical shaft furnace 10 has a feed hopper12 mounted thereon and connected thereto by a gas seal 14 into whichiron oxide, such as iron oxide pellets, lump ore, or similar material isfed. The force of gravity causes the iron oxide to flow into the shaftfurnace through the gas seal to form a packed bed burden 18 within theshaft furnace. As is conventional, metallized iron, preferably in theform of pellets or lumps, descends through the furnace, exiting througha lower gas seal 19 and discharge pipe 20, and is removed by conveyor22. Adding particulate iron oxide material to the hopper 12 establishesa gravitational flow through the shaft furnace 10 comprising an input ofiron oxide material and an output of particulate metallized iron 24.

The furnace 10 is surrounded by a bustle and tuyere system, generallyillustrated at 26, through which reducing gas is introduced into thefurnace 10. The portion of the furnace from the bustle and tuyere system26 to the top of the burden 18 constitutes the reducing zone. The lowerportion of the furnace contains a lower cooling gas distributor 30, andan upper cooling gas collector 32 which defines a cooling zonetherebetween. Collector 32 and distributor 30 are connected to gascooler 34 and pump/compressor 36 in a closed loop for recirculating thecooling gas through the cooling zone. Reacted or spent reducing gas isremoved from plenum 38 in the upper portion of the furnace 10 through agas take off pipe 40. The take-off pipe 40, through which spent reducinggas leaves the furnace feeds gas to a cooler scrubber 44. The coolerscrubber 44 can be a conventional venturi scrubber surrounded by anannular or sectional packed bed cooler and employing a recirculatingscrub water system. The spent reducing gas flows downward through theventuri and upwardly through the packed bed through which scrub waterdescends, providing intimate contact of the spent reducing gas with thescrub water.

Waste (used) scrub water is removed from the cooler scrubber 44 alongwith entrained particulates as underflow 46. Particulate materialremoved from the waste water is discharged as solid waste and removedusing any convenient means.

Cooler scrubber 44 is coupled through pipe 48 to the inlet of acatalyst-containing reformer 50. A spray nozzle 52 in the pipe 48injects an aqueous solution of chlorine dioxide into the pipe adjacentcompressor 54 to contact the cooled process gas directly, reacting withthe H₂ S therein to form a water-soluble sulfate. The nozzle 52 isconnected to a chlorine dioxide generator 56, which is in turn connectedto sources of sodium hypochlorite, sodium chlorite, hydrochloric acid,and water. Pump 57, connected to water cooling tower 59, provides thenecessary pressure to form the spray for injection. Meters M areprovided for monitoring and control of the process. The process gas withentrained sulfates is compressed in compressor 54, after which it passesthrough a baffle containing chamber or silencer 58, from which water andsulfates are discharged at 60. After passing throughhumidifier/aftercooler 62, the process gas is introduced to the reformer50. When using the alternate flow sheet (AFS) the humidifier/aftercooler62 chills the gas by utilizing cold water therein. In the standard flowsheet, the humidifier/aftercooler 62 acts as a humidifier of the gas byusing hot water, because some H₂ O is required for reforming of thespent reducing gas in the reformer. The humidifier/aftercooler also actsas a secondary scrubber.

Stoichiometric gas reformer 50 includes a plurality of indirect heatexchanger catalyst-containing tubes 66 positioned therein, which areheated by burners 68 using any convenient fuel. Hot reducing gas flowsfrom the catalyst tubes 66 of the reformer 50 to the bustle and tuyeresystem 26 through pipe 70.

The catalyst in tubes 66 in the reformer includes active ions such asnickel oxide which are believed to provide "active sites" in the coursesurfaces of the catalyst carrier. Catalyzation is thought to take placeat or near the active sites (see Roberton, AFP Catalyst of Gas Reactionby Metal, Logas, (1970). Sulfur compounds mask or reduce the activity ofthese active sites. An excess of sulfur adhering to the sites in thecatalyst carrier will reduce catalyst effectiveness by reducing the areaor number of active sites. Thus, the presence of sulfur can poison thecatalyst. The disclosed process and apparatus removes sulfur from thespent reduction gas thereby reducing the sulfur contamination of theactive sites of the catalyst. Reducing the sulfur contamination of thecatalyst improves the efficiency of the process.

It has been demonstrated that the system illustrated in FIG. 1substantially lowers the sulfur content of the spent process or reducinggas, the result being less sulfur contamination of the iron productdischarged from the furnace.

ALTERNATIVE EMBODIMENT

FIG. 2 illustrates an alternate embodiment of the invention. Thisembodiment is essentially identical with the embodiment previouslydiscussed with reference to FIG. 1, except that the sulfur-free processgas is first introduced into the cooling zone of the furnace throughline 74 as cooling gas, then is introduced to the reformer.

In this embodiment, the sulfur is removed from the process gas in thesame manner as the embodiment of FIG. 1, thus leaving the cooled productsubstantially sulfur-free, as no sulfur is present in the cooling gasfor removal by the hot pellets.

EXAMPLE

Reagents in the amount of 25% by weight of sodium chlorite, 12% byweight of sodium hypochlorite, and 36% by weight of hydrochloric acidwere fed to a chlorine dioxide generator in a feed mix of about 1/1/0.2,mixed with water, then the resulting solution was injected directly intothe spent reducing gas in pipe 48 at or near the gas entrance tocompressor 54. It was found that about 3.17 pounds of chlorine dioxideis used per pound of hydrogen sulfide, but it can vary from about 2.1 toabout 3.9 pounds ClO₂ per pound of H₂ S. This is substantially less thanthe 4.9 pounds chlorine dioxide per pound of hydrogen sulfide expectedto be required from the teachings of available literature.

"Pellets" as used throughout this specification and claims means:pellets, lumps, nodules, pieces, granules, agglomerated particulates,and the like, which are small enough to flow through a direct reductionfurnace and to have sufficient surface area to react with reducing gaspassing therethrough, yet large enough to support the weight of theburden. Finely divided material or fines, which would plug theinterstices between the pellets, blocking the gas passageway aroundthem, are not included in the definition of pellets.

"Metallized" as used throughout this specification, does not mean coatedwith metal, but means nearly completely reduced to the metallic state,i.e., always in excess of 60% metal, and usually in excess of 80% metalin the material. Such metallized iron in many forms, including pelletsand lump, is well suited as feed material to steelmaking furnaces suchas an electric arc furnace.

Although the invention has been disclosed and described with respect toa process for the direct reduction of iron oxide to form metallizediron, other metal oxide ores such as nickel or cobalt oxide, forexample, can be reduced to the metallized state using the disclosedprocess and apparatus. All such alternative uses of the disclosedprocess and apparatus are within the scope of the invention.

SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION

From the foregoing, it is readily apparent that I have invented animproved method and apparatus for removing sulfur compounds from spentprocess gas in the direct reduction of sulfur-containing metal oreswhich results in a low-sulfur metallized product, and which reducessulfur contamination of catalyst in an associated stoichiometricreformer.

It is to be understood that the foregoing description and specificembodiments are merely illustrative of the best mode of the inventionand the principles thereof, and that various modifications and additionsmay be made to the apparatus by those skilled in the art, withoutdeparting from the spirit and scope of this invention, which istherefore understood to be limited only by the scope of the appendedclaims.

What is claimed is:
 1. In a process for producing metallized pellets bydirect reduction of metal oxide, the method including the steps ofpassing a reducing gas through a furnace having an upper reducing zoneand a lower discharge zone, and containing sulfur-containing metal oxidematerial therein, reacting said reducing gas with said metal oxidematerial to produce metallized pellets and a sulfur-containing spentreducing gas, removing said spent reducing gas containing sulfur fromsaid furnace, cooling and scrubbing said spent reducing gas to formcooled spent reducing gas, reforming said spent reducing gas into aneffective reducing gas, and introducing said reducing gas into saidreducing zone;the improvement comprising the additional steps ofintroducing chlorine dioxide into said cooled and scrubbed reducing gas,reacting the sulfur containing components of said spent reducing gaswith chlorine dioxide, thereby reducing the sulfur content of saidreducing gas, removing the sulfur-containing reaction products, andproducing a cooled sulfur free gas stream.
 2. A process according toclaim 1, wherein the chlorine dioxide is in aqueous solution.
 3. Aprocess according to claim 2, wherein the sulfur-containing componentsin the spent reducing gas react with chlorine dioxide to form solublecompounds, further comprising removing the soluble compounds from thegas stream in aqueous solution.
 4. A process according to claim 3,further comprising compressing the spent reducing gas prior to removingthe soluble compounds.
 5. A process according to claim 1, introducingthe cooled sulfur free gas stream into the discharge zone.
 6. A processaccording to claim 3, wherein the soluble compounds are sulfates.
 7. Aprocess according to claim 1, further comprising passing the cooledsulfur free gas stream through the discharge zone as cooling gas tocontact and cool said metallized pellets prior to reforming said spentreducing gas.
 8. A process according to claim 1 wherein said metal isselected from the group consisting of iron, nickel and cobalt. 9.Apparatus for the direct reduction of metal oxide pellets to formmetallized pellets, comprising:a) a shaft furnace having an upper andlower end, said shaft furnace having means for charging metal oxide andcarrying a gravitationally descending burden therein; b) means forintroducing a reducing gas into said furnace to react with said burdento produce spent reducing gas and metallized pellets; c) spent processgas removal means at the upper end of said furnace; d) cooler-scrubbermeans communicating with said spent reducing gas removal means forcooling and scrubbing said spent reducing gas; e) means for injectingchlorine dioxide into the spent reducing gas to reduce the sulfurcontent of said spent reducing gas; and f) means for recycling saidspent reducing gas through a gas reformer to produce said reducing gas.10. Apparatus according to claim 9, wherein said furnace has a lowercooling zone, further comprising means for passing said cooled andscrubbed low sulfur spent reducing gas through said cooling zone ascooling gas.
 11. Apparatus according to claim 9, further comprisingmeans for compressing said spent reducing gas, wherein said means forinjecting chlorine dioxide into the spent reducing gas communicates withsaid compressor means.
 12. Apparatus according to claim 10 furthercomprising additional means for cooling the reduced sulfur content spentreducing gas.
 13. Apparatus according to claim 10 further comprisingmeans for humidifying the reduced sulfur content spent reducing gas. 14.Apparatus according to claim 9 further comprising means for intimatelycontacting said injected chlorine dioxide with said spent reducing gas,and means for removal of water and water-soluble products therefrom. 15.Apparatus according to claim 9 further comprising means for monitoringthe content of residual chlorine dioxide and residual sulfides in saidspent reducing gas downstream from said chlorine dioxide injectionmeans.